The present application relates to a nonaqueous electrolyte secondary battery. More specifically, the present application relates to a nonaqueous electrolyte secondary battery with high-power characteristics.
Recently, a lot of portable electronic equipment such as camera-integrated VTRs (videotape recorder), cellular phones, or notebook computers has appeared and it is contemplated to reduce the size and weight thereof. In response to the need, as an power supply of these electronic equipment, development for a secondary battery that is lightweight and capable of obtaining a high energy density has been proceeding. As the secondary battery capable of obtaining a high energy density, for example, a lithium-ion secondary battery is known.
In recent years, various researches have been made to improve characteristics of the lithium-ion secondary battery. The research on high-power characteristics is one of the most intensely studied research areas. For example, in Japanese Patent Application Laid-Open (JP-A) No. 2001-126760, a high-power and long-life battery has been developed by using ethylene carbonate, propylene carbonate, and dimethyl carbonate as solvents. For this reason, the lithium-ion secondary battery has been widely used for other applications including portable electronic equipment such as cellular phones or laptop computers. In the past, for example, a nickel-cadmium battery that is heavy and large and a nickel hydrogen secondary battery have been used for equipment that discharges a large current (e.g. power tools). In place of these batteries, the lithium-ion secondary battery that is small and lightweight has recently been used. Further, electrical vehicles and hybrid vehicles have recently been attracting attention from an environmental point of view. The use of the lithium-ion secondary battery as the battery for these vehicles has been considered. In the battery of such an automotive application, it is necessary to further emphasize the safety because a large current is discharged.
Incidentally, lithium-ion secondary batteries in the past are equipped with a Positive Temperature Coefficient (PTC element) to ensure safety (for example, refer to JP-A Nos. 2004-206959 and 10-144353). The PTC element has a feature that the resistance value of the PTC element is increased to block an electric current flowing into the battery when a large electric current flows into the battery or the temperature of the battery is higher. Thus, in the battery having the PTC element, external short circuit is generated and the PTC element operates to stop an electric current flow of the battery when the temperature of the battery reaches a predetermined temperature. As a result, abnormal generation of heat of the battery caused by a large current flow is prevented and the safety of the battery is ensured.